In animal husbandry, drug cross-contamination of animal feeds in feed mixing processes is a serious problem. Such drug carry-over is known to occur even when good manufacturing practices are followed.
Since sulfonamides are often used as poultry food additives, they are often involved in cross-contamination problems. For example, sulfadimethoxine [N'(2,6-dimethoxy-4-pyrimidinyl)sulfanilamide] is effective; both alone and in combination with a potentiator, e.g. 2,4-diamino-5(4', 5'-dimethoxy-2'-methylbenzyl)pyrimidine; in poultry feeds as an anticoccidial and antibacterial agent.
There is, thus, a need for a rapid method of detecting and estimating low levels of sulfadimethoxine in poultry feeds and, especially, in withdrawal feeds, i.e. feeds not containing the sulfadimethoxine but prepared using feed processing equipment which had been used for the preparation of sulfadimethoxine-containing feedstuff.
Such tests must also be suitable for use in detecting low-levels of sulfonamides in poultry feeds containing other feed additives such as, for example, amprolium, bacitracin, clopidol, erythromycin, epronidazols, lasalocid sodium (Avatec.RTM.), monensin sodium (Coban.RTM.), ormethoprim, trimethoprim and the like.
The Bratton-Marshall reaction is used in many assay procedures to determine sulfa drugs in poultry feeds. After extraction of the sample and purification of the extract, the extract containing a sulfa drug is then subjected to the Bratton-Marshall reaction. The Bratton-Marshall reaction consists of:
(a) acidification of the extract;
(b) reaction of the primary amine of a sulfonamide with a nitrite;
(c) decomposition of excess nitrite with sulfamic acid; and
(d) reaction of the diazotized amine with a coupling reagent (a chromogen, e.g., N-(1-naphthyl)-ethylenediamine dihydrochloride, to form a wine-red colored complex and
(e) determination of the sulfonamide concentration from the resulting colored complex.
This test, since it involves extraction and purification of an extract with subsequent acidification, diazotization, decomposition of excess nitrite and coupling is too elaborate and unwidely for a rapid field test.
A test suitable for rapid detection of a sulfonamide in the field involves the colorimetric reaction of Ehrlich reagent (an acidified methanolic solution of a chromogen, p-dimethylamino-benzaldehyde) with a sulfonamide. The Ehrlich reagent, which is also used as a spray reagent in thin layer chromatography (TLC), forms mostly yellow-colored Schiff bases upon reaction with the primary amines of sulfa drugs and/or their degradation products.
However, in this test, a yellow color can also result from interfering substances, e.g., other drugs or additives in feedstuff.
There, is, therefore, a definite need for a rapid, accurate test for sulfonamides in feedstuff which test is not deleteriously affected by the presence of interfering substances therein.